Acid
rain is rain that has been made acidic by certain
pollutants in the air. Acid rain is a type
of acid deposition, which can appear
in many forms. Wet deposition is rain, sleet,
snow, or fog that has become more acidic than
normal. Dry deposition is another form of acid
deposition, and this is when gases and dust particles become
acidic. Both wet and dry deposition can be
carried by the wind, sometimes for very long
distances. Acid deposition in wet and dry forms
falls on buildings, cars, and trees and can
make lakes acidic. Acid deposition in dry form
can be inhaled by people and can cause health
problems in some people.

What is acidity?
Acidic and basic are two ways that we describe
chemical compounds. Acidity is measured using a pH scale. A pH scale
runs from zero (the most acidic) to 14 (the most basic or alkaline).
A substance that is neither basic or acidic is called "neutral",
and this has a pH of 7.